In order to record information on optical disc media such as a DVD (digital versatile disc), it is necessary to generate a special write-operation waveform for suppressing interference of a write signal. Generation of such a special write-operation waveform requires highly accurate phase information that is finer than one-fortieth of the write data rate. However, it is difficult to realize such extremely fine phase accuracy by a single inverter chain (a ring oscillator), because the phase delay thereof is shorter than a delay of a single inverter circuit. Therefore, conventionally, a plurality of ring oscillators are used and the respective inverter circuits in the ring oscillators are connected by phase-coupling circuits so as to slightly change the output phase of each ring oscillator, whereby phase information finer than that that can be generated by a single ring oscillator is produced (see, for example, Patent Documents 1, 2, and 3).
Also, a resistor ring, in which a plurality of resistors are connected, is provided, and the connection points of the resistors in the resistor ring are connected with the connection points of phase delay elements in a plurality of ring oscillators are connected to thereby produce fine phase information (see Patent Document 4, for example)
Patent Document 1: Specification of U.S. Pat. No. 550,030
Patent Document 2: Specification of U.S. Pat. No. 5,475,344
Patent Document 3: Specification of U.S. Pat. No. 5,717,362
Patent Document 4: Specification of US patent application publication No. 2006/0049879